A spark plug is used to ignite an internal combustion engine such as a vehicle engine. Generally, the spark plug includes a tubular metal shell, a tubular insulator, a center electrode, and a ground electrode. The insulator is arranged in an inner hole of this metal shell. The center electrode is arranged at the inner hole on a distal end side of the insulator. One end of the ground electrode is bonded to the distal end side of the metal shell. Between the other end of the ground electrode and the center electrode, a spark discharge gap is provided. The spark plug is spark-discharged at the spark discharge gap, which is formed between the distal end portion of the center electrode and the distal end portion of the ground electrode, in a combustion chamber of the internal combustion engine. Thus, the spark plug burns a fuel filled up in the combustion chamber.
As a material forming the center electrode and the ground electrode, a Ni alloy or a similar material is generally used. Regarding oxidation resistance and wear resistance, the Ni alloy is slightly inferior to a precious metal alloy whose main constituent is a precious metal such as Pt and Ir. However, because of its inexpensiveness compared with the precious metal, the Ni alloy is preferably used as the material forming the ground electrode and the center electrode.
Recently, there has been a trend toward high-temperatures in the combustion chamber. Therefore, if spark discharge occurs between the distal end portion of the ground electrode and the distal end portion of the center electrode made of the Ni alloy or a similar material, each distal end portion of the ground electrode and the center electrode opposed to one another may be likely to generate spark erosion. Therefore, the following methods have been developed. Tips are disposed at each distal end portion of the ground electrode and the center electrode opposed to one another to generate the spark discharge at the tips. Thus, the wear resistances of the ground electrode and the center electrode are improved.
As the material forming the tips, a material whose main constituent is a precious metal excellent in the oxidation resistance and spark erosion resistance is often used. The material includes Ir, an Ir alloy, a Pt alloy, or a similar material. To improve durability of the tip, there has been proposed a tip that includes a protective coating layer or a similar layer at a surface of a core material whose main constituent is Ir (for example, see JP-A-2004-31300 and JP-A-2012-38733).
Recently, in a spark plug, to ensure improving high output and fuel economy of an engine, there has been developed a direct-injection gasoline engine, a lean burn engine, or a similar engine. The direct-injection gasoline engine directly injects a fuel around a plug in a fuel chamber. The lean burn engine decreases a mixing ratio of the fuel to the air to allow burning under high oxygen concentration atmosphere.
Such engine requires a spark plug to have oxidation resistance. Accordingly, as a material that forms tips, which are formed at a center electrode and a ground electrode, a Pt alloy, which is known for excellent oxidation resistance, has been considered to ensure providing durability more than an Ir alloy, which is known for excellent spark erosion resistance. In particular, a Pt—Rh alloy, which is formed by containing Rh to Pt, where the oxidation resistance and the spark erosion resistance are improved has been considered as preferable.
However, assume the case of the use of a spark plug with a tip made of the Pt—Rh alloy for the direct-injection gasoline engine. In the direct-injection gasoline engine, intake air gas and fuel are likely to directly hit the tip surface in a lean burn state, which is the high oxygen concentration atmosphere. Accordingly, it has found that abnormal erosion may occur in a specific site at the tip, especially, a side surface on an intake charge mixture valve side of the tip. It has found that the abnormal erosion at the side surface of this tip is likely to occur in the following case. The spark plug is used in the combustion chamber at a high temperature and high oxygen concentration. Further, the tip is placed under conditions where the intake air gas and the fuel are likely to directly hit the surface of the tip and oxygen concentration and the temperature at the periphery of the tip widely change.
JP-A-2002-359050 discloses abnormal erosion occurred in the Ir alloy. The abnormal erosion at the side surface of the tip made of the Pt—Rh alloy differs from the abnormal erosion of JP-A-2002-359050 in a mechanism of the erosion. The precious metal tip containing Ir and 20 mass % of Rh, which is disclosed in JP-A-2002-359050, “generates abnormal erosion in the form along arc hollowing an outer peripheral side surface other than a discharge surface of a discharge portion” (paragraph number 0005). In contrast to this, the tip made of the Pt—Rh alloy generates the abnormal erosion in the form of like the one entire side surface of tip dissipating. The difference between both in the mechanism of wear can also be seen from a difference in a wear form of both.
An advantage of this invention is a spark plug featuring excellent durability. The spark plug includes a tip on at least one of a center electrode and a ground electrode. The spark plug with excellent durability can be achieved by reducing abnormal erosion of the tip.